In chemical production, there exist a large number of container devices for reaction or separation that use external circular heating, such as devices for producing polyesters, wherein the container may be a kettle or a tower. The raw materials to be heated generally enter an external heat exchanger from the bottom of the container through a pipe for heating, and then return to the container from the wall at the lower part of the container through the pipe. There exists a density difference of the materials before and after heating, which creates a driving force for the materials from the bottom of the container to automatically return to the container through the heat exchanger, said driving force being known as thermal siphon.
Polyesters mainly comprise poly diethyl p-phthalate, poly dibutyl p-phthalate, and dipropyl p-phthalate. Presently, their preparation processes are mostly direct esterification, i.e. carrying out esterification from phthalic acid and the corresponding diol directly. The existing processes for producing esters are mostly continuous ones. There are many technological routes, but they are basically composed of three continuous procedures, i.e. esterification, prepolycondensation, and polycondensation, wherein esterification is at the forefront and therefore is the most vital part of the whole technological route. It will exert significant effect on the operability of the subsequent procedures and the quality of the final product. The esterification device must meet the requirements of a heavy heat duty, uniform heating of the materials, instant removal of excessive diol and water generated in esterification to facilitate further reaction, and a flow of the materials in the system near the plug flow as much as possible to make the residence time of various parts of the materials distribute uniformly. The importance of the esterification reaction and its high requirements for the esterification device make the study of the esterification device become the most attractive part in the study of the polyester process.
In the prior art, the earlier esterification device was the continuous stir reactor, which uses a jacket or internal tube heat exchanger for heating and exerts violent stirring. In addition, series multiple kettles are used to improve the distribution of the residence time of the materials. The drawbacks of the continuous stir reactor are evident, i.e., the occurrence of the temperature field and concentration field of the internal materials caused by the amplification of the device, which affect the uniformity of the materials, a high investment resulted from large numbers of the devices and powerful stirring equipments, and a considerable operation cost of the stirring equipments. Latter developed externally circular heating esterification device made a great progress, such as the device introduced in JP Tokkai-Hei 10-87805 and JP Tokkai-Hei 11-116536. Such an externally circular heating esterification device mainly comprises an external tube heater and a reaction kettle. A solid/liquid two-phase mixed slurry of phthalic acid and diol is first injected into the circulating oligomer, then fed into a tube heater to complete heating and carry out primary esterification reaction, and finally fed into the reaction kettle for further esterification. The product is discharged from the bottom of the kettle, a part of which returns to the heater for circulation. Various baffle components are equipped in the reaction kettle, so that the stream in the kettle approaches the plug flow as much as possible. The merits of the externally circular heating esterification device are to better meet the need for powerful heat duty, to use no stirring equipment, and to make the residence time distribution of various parts of the materials more uniform.
However, such a prior externally circular heating esterification device has an evident drawback. The driving force for the stream only comes from the thermal siphon, but the flow rate is generally difficult to meet the requirement of the process only using the driving force generated by thermal siphon for the materials to automatically circularly flow. Installation of pumps in the pipe between the bottom of the container and the heat exchanger is a commonly used remedial method, but the use of pumps in chemical devices will not only increase the energy consumption, but also increase the routine maintenance cost. Another scheme for overcoming the drawback is to increase the loading of the diol in the reaction materials, the large amount of excessive diol serving as a carrier for the flow of the reactants. Taking the preparation of bis-hydroxyethyl p-phthalate (BHET) from p-phthalic acid (PTA) and ethylene glycol (EG) as an example, the molar ratio of feeds EG to PTA is generally 2.0-2.2:1, and the excessive diol increases unnecessary energy consumption from evaporation to recovery by condensation.